Honeycomb porous structures are used as filter bodies for filtering particles emitted by diesel vehicles. The filter bodies are usually made of ceramic (cordierite, silicon carbide, etc.). They can be monolithic or assembled from a plurality of blocks. In the latter case, the blocks are bonded together with a ceramic cement. The assembly is then machined to the required section, which is usually round or oval. The filter body can include a plurality of passages which are closed at one end or the other, can have different shapes and diameters in cross section, and is inserted into a metal casing, for example as described in FR-A-2 789 327.
After some time in use, soot accumulates in the filter body passages, in particular on the upstream face, which increases the head loss due to the filter body and therefore reduces the performance of the engine. For this reason the filter body must be regularly regenerated (for example every 500 kilometers).
Regeneration consists of oxidizing the soot. This requires heating, because the self-ignition temperature of the soot is of the order of 600° C. under the usual operating conditions, while the temperature of the exhaust gases is only of the order of 300° C. However, additives can be added to the fuel to catalyze the soot oxidation reaction and reduce the self-ignition temperature by approximately 150° C. The exhaust gases, the filter body or the soot can be heated. Various techniques have been developed but consume a great deal of energy and are very often difficult to control.
A recent and advantageous approach consists of localized heating ahead of the filter body to initiate combustion, which then propagates progressively to the whole of the filter body. This type of technique is described in FR-A-2 771 449 and DE-A 19530749, for example.
The means for heating particles deposited on the filter body are connected to an electrical power supply of the vehicle and consist of diesel engine preheater glow plugs, for example.
Such heating means have a number of drawbacks. First of all, they are bulky, which makes it difficult to position them relative to the filter body. FIG. 2 of FR-A-2 771 449 shows clearly that it is not possible to place the heating means in direct contact with the soot and even less so with the core of the filter body. Moreover, it is found that the presence of the heating means blocks access of the exhaust gases to a number of filter body passages, considerably reducing efficiency. Also, a great deal of energy is consumed and the regeneration system has a mediocre response time because the temperature increases relatively slowly.
Other heating means, such as simple electrical elements, are unsuitable because the temperatures can reach more than 1000° C. in the filter during combustion of the soot, and few materials can be used under these temperature and oxidation conditions because the problem of rapid wear due to corrosion becomes very serious.
There is therefore a need for heating means for particulate filters for purifying exhaust gases of internal combustion engines, in particular diesel engines, free of the drawbacks previously cited.
The invention aims to meet that need.